The present invention relates to an optical sensing system used in a device such as a video cassette recorder or deck having multiple operating modes.
FIG. 1 illustrates a conventional sensing system used in a video cassette recorder. The system comprises a power supply node 1 coupled to the anode of an infrared light-emitting diode (IR-LED) 2. The cathode of the IR-LED 2 is coupled through a resistor 3 to ground. The IR-LED 2 emits light toward a space through which a magnetic tape 4 travels when the video cassette recorder is operating. If not blocked by the magnetic tape 4, the light passes through a slit referred to as the start-sensor slit 5. On the far side of the slit, another power-supply node 6 is coupled to the collector of an infrared phototransistor 7, the emitter of which is coupled through another resistor 8 to ground. The voltage at the emitter electrode of the infrared phototransistor 7 is supplied as a sensor signal (SS) to a microcomputer 9 comprising an analog-to-digital converter (A-D) 10 and a central processing unit (CPU) 11. The microcomputer 9 also receives inputs from a power on-off switch 12, a rewind switch 13, and a deck mode sensor 14. The deck mode sensor 14 senses the operating mode of the video cassette recorder. The video cassette recorder has various operating modes, including play, fast-forward, rewind, and stop. The signal from the deck mode sensor 14 indicates whether the magnetic tape 4 is moving or stopped.
The sensor system in FIG. 1 operates as a start sensor, enabling the video cassette recorder to start operating automatically when a cassette is inserted. FIG. 2 illustrates a sequence of operations and the accompanying electrical input to the IR-LED 2 and electrical output (SS) of the infrared phototransistor 7. Mechanical modes and operations are indicated between the IR-LED and SS waveforms, with logical and electrical modes indicated in parentheses.
Initially, power is off (P-OFF) and no cassette is present. Even in this power-off mode, power (1.3 V) is supplied to the IR-LED 2, which emits infrared light continuously. Since no cassette is present, the cassette handling mechanism (not shown in the drawings) is in the eject position. This mechanism is linked to the start-sensor slit 5. In the eject position, the start-sensor slit 5 is closed, so the infrared phototransistor 7 does not receive the infrared light emitted by the IR-LED 2. Accordingly, the infrared phototransistor 7 does not conduct, and the sensor signal SS is at the ground level (0 V). In the microcomputer 9, which also continues to receive-power in the power-off mode, the analog-to-digital converter 10 converts the sensor signal SS to a digital value, which is read by the CPU 11.
At a time A, the user inserts a cassette, and the cassette-handling mechanism mechanically opens the start-sensor slit 5. The infrared phototransistor 7 now receives the infrared light emitted by the IR-LED 2 and conducts, causing the sensor signal SS to go to a higher level such as five volts (5 V). At first, the video cassette remains in the power-off mode, as indicated by the arrow in parentheses. Shortly after detecting the SS signal transition, however, the microcomputer 9 powers up the other circuits of the video cassette recorder, entering the power-on mode (P-ON), and initiates a slot-in operation that mechanically loads the magnetic tape 4 into position for recording and playing. In this position the magnetic tape 4 is disposed between the IR-LED 2 and start-sensor slit 5, but the magnetic tape 4 begins and ends with a transparent leader section, lacking a magnetic coating. It will be assumed below that the transparent leader section at the end of the magnetic tape 4 is loaded into the position between the IR-LED 2 and start-sensor slit 5, so the infrared phototransistor 7 continues to receive light and the sensor signal SS remains high.
During the slot-in operation, the user presses the rewind switch 13. When the slot-in operation is completed at time B, the microcomputer 9 immediately initiates a rewinding operation (REW), causing the magnetic tape 4 to be rewound by a motor not shown in the drawing. The opaque magnetic tape 4 now blocks the light emitted by the IR-LED 2, and the sensor signal SS goes low. When the transparent leader section (LDR) at the beginning of the magnetic tape 4 reaches the start-sensor slit 5, however, the infrared phototransistor 7 receives the infrared light emitted by the IR-LED 2, and the sensor signal SS goes high again. This signal transition is detected by the analog-to-digital converter 10 and CPU 11 in the microcomputer 9, which reacts by stopping the rewinding operation. To avoid mechanical stress and strain on the cassette, tape, and motor, the sensor system must detect the leader tape quickly and the microcomputer 9 must stop the rewinding operation promptly.
At point C, the user presses the power on-off switch 12 to switch power off. The microcomputer 9 switches off the power supplied to the infrared phototransistor 7, and the sensor signal SS goes low.
As shown in FIG. 2, the IR-LED 2 is left on at all times. As a result, the IR-LED 2 consumes power unnecessarily when the video cassette recorder is not being used. Over the lifetime of the video cassette recorder, much power is dissipated in this way. This power dissipation could be reduced by operating the IR-LED 2 in an intermittent or pulsed manner, but that would lead to undesirable delays in detecting the leader tape in high-speed operations such as fast-forward and rewind.
An object of the present invention to reduce power dissipation in a sensor system that optically senses the presence of a recording medium, without slowing the response of the sensor system in high-speed operation modes.
The invented sensor system has a light-emitting element, a switching element supplying power to the light-emitting element, and a light-sensing element. The light-sensing element receives the light emitted by the light-emitting element, conditional on the presence of an object, and converts the received light to an electrical signal indicating the presence or absence of the object.
The sensor system is used in an apparatus having a plurality of operating modes. A control unit of the sensor system controls the switching element, causing the light-emitting element to emit light continuously in one operating mode of the apparatus, and in a pulsed manner in another operating mode of said apparatus.
If the apparatus is a video cassette recorder and the object to be sensed is a magnetic tape, continuous light emission can be used in fast-forward and rewind modes for rapid sensing, while pulsed light emission is used in a power-off mode and a power-save mode to save power. Pulsed light emission can also be used in the record, play, and stop modes.